He taught himself the instrument, picking up Jeff Buckley, Radiohead and Bob Dylan songs by ear until, one day, he nervously entered a school music contest.

"Everyone else was singing Pie Jesu, but I brought my guitar and I was like, 'I'm going to do [Bob Dylan's] It Ain't Me Babe,'" he recalls.

But halfway through his performance, the guest judge ("some famous cellist, I think") interrupted.

"He puts his hand up and goes, 'OK, that's enough'.

"And there's an audience with all my friends and peers - and I just went, 'Yo! Don't interrupt me and my flow, bro!' I slammed my guitar on the floor and stormed out.

Pivotal moment

"I don't know what came over me, because I'm not that kind of guy, but in that moment I became enraged. I walked out of the school entirely and I was crying on the phone to my mum.

"But, long story short, the judge ended up saying the reason he cut me off was because I was going to win - and I ended up going to the final and winning the whole competition."

It was a pivotal moment for the young singer. One that made him double-down on his ambition to pursue music.

He retreated to his bedroom ("it's got a low-hanging ceiling like a hutch") and started making demos on Garage Band, using a "terrible" USB microphone and drawing inspiration from lyrics he'd scrawled across the walls.

One of his first compositions was a rusty, intimate ballad called Last Words. Gracie posted it on Soundcloud, where it immediately caught people's attention.

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Media captionIsaac Gracie performs Last Words for BBC Music Introducing.

It caused such a stir, in fact, that the head of Universal Music flew from LA to see Gracie's first London show. He was quickly signed to Virgin EMI and dropped out of his creative writing course at the University of East Anglia. But his head was spinning.

"I was the opposite to being prepared," he says. "I didn't know where the road was taking me - but I also didn't know that the road was even open to people like me, to ordinary people.

"For some reason, I thought anyone who was successful in music came from a different realm of existence."

The dissonance triggered a crisis of confidence. Gracie started comparing his "rough and awful-sounding" demos to the singers he idolised.

"I thought I was a fake, you know? All of a sudden I hit a wave of inertia and self-doubt and depression that I'd never really experienced before.

"The terror of it all started coming over me."

To make things worse, the sudden acceleration of Gracie's career tore him away from his girlfriend, and they eventually broke up.

"It completely twisted our relation not only to each other but also to the world," he says. "Because all of a sudden I had to disappear and do this stuff and she had to watch me go.

"Because life pulled us apart, rather than us deciding we were going to separate, there was a lingering sense of unfairness."

Image copyrightIsaac Gracie / InstagramImage caption
Gracie was called "a messianic Macaulay Culkin" in a recent review. "I can kind of see why," he laughs, "but they put it in the frickin' headline".

That relationship, and the rubble of its remains, inspired most of Gracie's subsequent songs, from the contemplative Silhouettes of You to the angrier, desolate Death Of You & I.

The singer never absolves himself of blame. "I've never given so little and promised so much," he sings on When You Go; while admitting he "faked interest" in his girlfriend's stories on the flute-assisted One Night.

He says his relationships are haunted by the sins of his father, who deserted the family when Gracie was young - and whom he hasn't seen for three years.

"I would have said all of those songs were about my ex-girlfriend but in many respects my mum and my dad exist in them as well," he explains.

"A lot of the songs are about block emotions of abandonment or guilt or heartbreak."

The singer's insecurities surfaces in other ways on Terrified, which was written as a riposte to his own hype.

Those feelings caught up with him during the two-year creation of his debut album.

"I set moronically high expectations for myself," he laughs. "I wanted it to be the best record of all time."

Key to the problem was that he had to re-record those bedroom demos without diluting their essence. Last Word, in particular, was revised and re-versioned several times.

"The song is like a little hymn," he explains, "so you can't just say, 'Let's produce it like Hold Back The River' because it won't work on those terms."

The first attempt, recorded by Markus Dravs (Arcade Fire, Florence + The Machine), bludgeoned the song's delicate beauty, launching into the first verse with a double kick drum and starving Gracie's angelic vocals of oxygen.

In the end, the singer went back to the arrangement of the original demo, adding subtle embellishments that combust in a cathartic climax which puts his choral training to excellent use.

"I only realised fairly recently how much choral music had played a role in how I record songs," he says. I really try to carry through the evocative, emotional anguish."

Now that the finished version of Last Words is out in the real world, the singer is finally satisfied.

"I really love it," he says. "It's a great song and everyone should listen to it - but I'm also going to say that it was a frickin' trial and all the pressure and stress I felt boiled down to that one song."

Getting out of the studio and playing live has finally restored Gracie's confidence.

Taking to the stage in London earlier this year, he was in playful mood - "Is a handsome nipple showing?" he asked the audience, tugging at his unbuttoned shirt. "No it isn't. I'm sorry."

During The Death of You & I, his long, dirty-blonde hair explodes around him in a thrash of guitar noise the youngster would never have contemplated in his childhood bedroom ("we've got neighbours!" he protests)

"Do I look forward to that part of the set? Oh hell, yeah!" he grins, in a rare moment of eye contact.

"Playing with the band made me realise I enjoy singing these songs, and I enjoy seeing the reaction people are having - and therefore they must have merit.

"Now I have a desire, a real drive, to get back in the studio.

"I'm sure all of those questions - all those emotions and confrontations with myself - will come back up again. But now I've got a roadmap for where I want to go."